Dense cloud cores are condensations in star-forming molecular clouds. Their gas mass is typically a few stellar masses, and they are frequently observed to harbor protostars, the youngest known stars. They are widely believed to be the birth sites of stars, and their physical properties are believed to represent the "initial conditions" for star formation.

Dense cores are detected via the emission or absorption by their high column densities of dust and gas. Their dust grains absorb the emission of background starlight at optical and infrared wavelengths, and these same grains emit detectable thermal radiation at infrared, submillimeter and millimeter wavelengths. Cores harbor a wide variety of molecular species, many of which radiate in rotational lines at centimeter, millimeter, and submillimeter wavelengths. With sensitive observations of dense cores, it is possible to estimate core mass, density, temperature, chemical abundances, and to characterize their magnetic field strengths, turbulence, and internal motions.

The dense starless globule Barnard 68, imaged in six different wavebands using the FORS1 camera at the VLT (0.44, 0.55 and 0.90 micron) and the SOFI camera at the NTT (1.25, 1.65 and 2.16 micron) by Joao Alves, Charles Lada, and Elizabeth Lada. The extinction of the background stars caused by the globule decreases with increasing wavelength - causing the "core's shadow" to disappear.
Figure based on ESO press release photos 29a-b/99 - for further details see: ESO press release.